This invention relates to a process for the production of metal salts of hydrocarbyl dithiophosphoric acid particularly, overbased metal salts of hydrocarbyl dithiophosphoric acid.
It is well known that various additives can be used in lubricating oils in order to improve certain oil properties and to make a more satisfactory lubricant. For example, antiwear agents are intended to decrease wear of machine parts. Wear inhibitors for incorporation in motor oils and industrial oils are finding greater use as a result of greater stress placed on moving parts in high performance engines. Numerous other additives have been developed for use in such oil compositions to improve the lubricating characteristics thereof and thereby to lessen the wear of the moving parts.
Of the antiwear agents, the metal salts of hydrocarbyl dithiophosphoric acid, such as the diaryl and dialkyl dithiophosphates, especially zinc dithiophosphates, have long been used as antiwear additives and antioxidants in hydraulic oils, motor oils, automatic transmission fluids and the like. Processes for the production of metal salts of hydrocarbyl dithiophosphoric acid are well known. See U.S. Pat. Nos. 2,838,555; 3,848,032; 4,085,053; 4,123,370; 4,215,067; and 4,263,150 incorporated herein by reference. In a typical reaction, four equivalents of a hydroxy compound are reacted with phosphorus pentasulfide in the presence of a catalyst. Once formed, the hydrocarbyl dithiophosphoric acid is then separated from the reaction mass, and subsequently neutralized with an excess of metal base such as zinc oxide.
An important characteristic in determining the antiwear properties of the overbased metal salt of hydrocarbyl dithiophosphoric acid is the metal to phosphorus ratio. Typically, the metal to phosphorus ratio should be no less than about 1.15:1 and most preferably greater than about 1.2:1. However, variations in reactants and process conditions result in undesirable variations in the metal to phosphorus ratios of the products thus formed. If the metal salt of dithiophosphoric acid has too low a metal to phosphorus ratio, blending of the reaction product with a product having a higher metal to phosphorus ratio is required. It is desirable therefore to provide a process which will more consistently result in a clear reaction product having a metal to phosphorus ratio of no less than about 1.15:1.